1. Field of the Invention
The invention relates to a discharge lamp of the short arc type.
2. Description of the Prior Art
For a photolithography process in semiconductor manufacture, for example, among discharge lamps of the short arc type, high pressure mercury vapor lamps of the short arc type are used. Recently there has been a demand for narrowing emission distribution spectral bands of ultraviolet rays which involve mainly 436 nm (hereinafter called the "g-line") or 365 nm (hereinafter called the "i-line") in order to increase image resolution upon exposure with ultraviolet rays.
To narrow the spectral width of the g-line or i-line, it is a good idea to reduce the mercury pressure within the arc tube of a mercury vapor lamp of the short arc type. This means that, in the case of high mercury pressure, the pressure expansion of the i-line spectrum increases, by which the spectral width of the g-line or the i-line expands. If, on the other hand, the mercury pressure is low, the pressure expansion of the i-line spectrum diminishes, by which narrowing of the spectral width of the g-line or the i-line is enabled. If, however, the mercury pressure within the arc tube is reduced, the electrical resistance between the electrodes drops, by which the lamp voltage decreases during luminous operation. As a result thereof, the lamp current in the case of the same lamp output power is multiplied.
If the lamp current is multiplied in this way, the anode temperature increases even more as a result of collision of the multiplied electron current. Furthermore, the effect is to suppress the vaporization of the anode material by the above described mercury because the mercury pressure within the arc tube is low. Consequently, the disadvantages were premature vaporization of the anode, blackening of the inside wall of the arc tube and shortening of the service life.
For example, Japanese patent disclosure document HEI 2-256150 discloses a process for suppressing this vaporization of the anode part. Here, it is disclosed that the anode surface outside its tip includes porous layers consisting of a mixture of tantalum carbide and tungsten in order to reduce the anode temperature during luminous operation of the lamp. Further, the disclosure discusses increasing the effective surface area of the anode, multiplying the radiation from the above described surface, and, thus, reducing the anode temperature, and the like.
Here the tantalum carbide due to its higher radiation capacity than tungsten is therefore also used to increase the radiation effect. But tantalum carbide has the property of decomposing by reduction in an area with a high temperature in the vicinity of the electrode tips. This means that for this reason the area in which the mixed layers could be formed was limited, although actually the mixed layers of the tantalum carbide and tungsten should be formed into near the above described tip to cool the temperature of the anode tip which has the highest temperature.
On the other hand, there is a demand for increasing even more the degree to which the illumination intensity of the g-line or the i-line is maintained in the above described lithography process. Here there is increased demand for preventing blackening of the tube wall of the discharge lamp of the short arc type compared to what was conventionally the case.
The disadvantage of vaporizing the electrode material resulting from temperature increases of the electrode, however, is also present in other discharge lamps of the short arc type, for example, in a xenon lamp or the like.